This invention relates to mixing machines for foundry mold sand and the like with increased efficiency and yet a simple and rugged construction.
The desire to expose all of the sand in a mixing vessel maximally frequently and uniformly to the mixing tools has resulted in two types of structure for container mixers. In one type of structure, the container is stationary and eccentric mixing tools move along paths about the container axis of the mixer during mixing operations. In the other type of structure, the mixing container rotates and the eccentric mixing tools or their respective shafts, if driven, are stationary. Theoretically, in the first type of arrangement, the sand is at a standstill with respect to the container and the eccentric mixing tools are moved by the stationary sand; and in the second type of structure, the shaft of the rotating eccentric tool is stationary and the sand is entrained by the rotating bowl and moved toward the eccentric tools. Therefore, in these mixers, an important feature is that the sand does not execute a relative motion with respect to the bowl. Thus, the sand is supposed to be also at a standstill, with the bowl being stationary, and to participate in the rotation, if the bowl is rotating. In order to enhance this behavior, vertical ribs have in many cases been provided along the periphery of the bowl, in order to brake the sand, or the bowl circumference has been equipped with a high friction by an appropriate selection of the material, for example by lining the bowl with rubber. In conventional mold sand mixers with eccentrically disposed tools, it has furthermore been necessary to provide, in addition to the drive for rotating the eccentrically disposed tools about their own axis, a further drive means, either for the movement of the shafts of the eccentrically disposed tools about the bowl axis or for the rotation of the bowl, whereby these machines become complicated and expensive in manufacture and upkeep. Furthermore, at higher speeds of rotation, difficulties are encountered in the central discharging of rotating mixing containers due to centrifugal effects. Also, when the container is at a standstill and the mixing tools rotate in a planetary motion, the drive is effected from above, whereby the introduction of the sand is impeded.
The present invention contemplates providing a mixer which avoids the above-discussed disadvantages and, with a simple construction and correspondingly low manufacturing costs, shows a high mixing efficiency. More particularly, the present invention contemplates providing a mixer having a stationary mixing bowl having a rotating central tool disposed at the bottom thereof for imparting rotational movement to molding sand or the like being mixed. At least one driven centrifugal tool which is spaced from the axis of the bowl extends downwardly into the bowl from above facing spaced relationship to the central tool for imparting further mixing motion to the material in the bowl. According to the invention, each of the centrifugal tools are disposed at fixed positions around the periphery of the staionary bowl, thereby obviating the need for mechanisms to rotate either the bowl or the centrifugal tools about the bowl axis. In preferred embodiments according to the invention, the rotating central tool imparts maximal rapid rotation to the contents of the container.
According to a further feature of the invention, the bowl is provided with a smooth inner surface made of a material with low friction so as to not retard the rotational motion of the bowl contents. In particularly preferred arrangement of the present invention, the inner surface of the bowl, along the walls and the bottom, is provided with a lining of hard-chromeplated steel sheet.
According to another feature of a preferred embodiment of the present invention, the central tool is provided with a plurality of agitator vanes extending radially from a central hub portion to immediately adjacent the vertical wall of the bowl. A plurality of centrifugal tools are also provided which are arranged spaced from one another and from the bowl axis which coincides with the rotational axis of the central tool. The centrifugal tools and the agitator vane are provided at positions with respect to one another such that only a single centrifugal tool faces any of the vanes at any given time. That is, at any given time during operation of the mixer with the central tool rotating such that the vanes sequentially pass under the centrifugal tools, only a single centrifugal tool is directly vertically above a vane moving thereunder at any given time, with all other centrifugal tools being vertically above a space intermediate the vanes at said given time.
According to another feature of the invention, the centrifugal tools are rotated oppositely to the direction of rotation of the agitator vanes of the central tool. Another feature of a preferred embodiment of the invention is that the axes of the centrifugal tools deviate from the vertical so that their spacing from the central axis of the bowl (also rotational axis of the central tool) is larger at the top than at the bottom. The centrifugal tools are configured and spaced with respect to the bowl so as to engage the top layer of sand during mixing operations. Another feature of a preferred embodiment of the invention is in the inclusion of a plurality of obliquely oriented, exchangeable vane plates on each of the agitator vanes, which vane plates are spaced from one another along the length of the vanes from the shell or wall of the mixing bowl to the central drive hub.
These and further objects, features and advantages of the present invention will become more obvious from the following description when taken in connection with the accompanying drawings which show, for purposes of illustration only, a single embodiment in accordance with the present invention.